In recent years, materials comprised chiefly of hydrophilic polymers have been widely employed in artificial organs, artificial muscle, medicinal drug carriers, cellular scaffolding materials, agricultural materials (moisture-retaining agents), and the like.
Examples of natural hydrophilic polymers include alginates, collagen, hyaluronic acid, chondroitin sulfate, fibrin, chitosan, and silk. These polymers are of high biocompatibility, but present problems in the form of high biodegradability and low mechanical strength. Contamination by pathogens is also a concern.
Examples of artificially synthesized hydrophilic polymers include polyethylene glycol, polysodium acrylate, polyacrylamide, polylactic acid, polyhydroxyethyl methacrylate, and polyacrylamidomethylpropane sulfuric acid. These polymers are of high mechanical strength and present little risk of contamination by pathogens. However, they present a problem in the form of low biocompatibility. There is also a problem in that some of these polymers are highly toxic.
In recent years, a polymer material that is highly biocompatible and hydrophilic has been provided by employing 2-methacryloyloxyethyl phosphorylcholine (MPC) (Collected Papers on Polymers, 1978, Vol. 35, No. 7, pp. 423-427; and Japanese Patent No. 2870727, the disclosures of which are expressly incorporated by reference herein in their entireties). The phosphatidyl choline moiety in polymers manufactured with MPC has a structure similar to that of phospholipids, which are compounds found within the body. The fact that this moiety is charge neutral and highly hydrophilic is a factor that increases its biocompatibility.
However, the high solubility in water of 2-methacryloyloxyethyl phosphorylcholine complicates synthesis. Polymers comprising a high ratio of 2-methacryloyloxyethyl phosphorylcholine present problems in that they are too hydrophilic to form particles in water, and have low mechanical strength.
Recently, drug delivery technology has been employed to selectively accumulate imaging agents in affected areas in an attempt to heighten the contrast of images of affected areas and reduce the quantity of imaging agent administered.
For example, the encasing of a drug or the like in the form of an imaging agent within a particle such as a liposome or micelle and the modification of the surface of the particle with a ligand molecule or the like, and the coating of drugs with polymers, have been reported (U.S. Pat. Nos. 5,686,061 and 5,019,370; International Patent Application Publication No. 06106513; U.S. Patent Application Publication Nos. 2007098640, 2007098641, and 2007098642; J. Am. Chem. Soc. 2000, 122, 8940-8945; J. Control. Release 2007, 122, 269-277; and Nature Medicine 2007, 13, 636-641, the disclosures of which are expressly incorporated by reference herein in their entireties). However, such imaging agents have low stability and low safety, and large particle size, and tend to be captured by the reticuloendothelial system, and the like, thereby compromising imaging performance.
Iodine-containing compounds, for example, are known X-ray imaging agents. Triiode benzenes are employed in vascular imaging and urethrography. However, most X-ray imaging agents are water-soluble compounds of low molecular weight, and do not remain in the blood long following administration. Thus, imaging must be conducted immediately after administering the imaging agent. Toxicity when a large quantity of imaging agent is employed is frequently reported (Toxicology 2005, 209, 185-187, the disclosure of which is expressly incorporated by reference herein in its entirety).